1. Field of the Invention
The present invention relates generally to a non-aqueous electrolyte battery comprising an anode, a cathode using a carbon material, and a non-aqueous electrolytic solution, and more particularly, to a non-aqueous electrolyte battery whose preservation characteristics and cycle characteristics are improved upon improvement of the cathode using the carbon material.
2. Description of the Prior Art
In recent years, as one of new-type batteries having high power and high energy density, a high electromotive-force non-aqueous electrolyte battery using a non-aqueous electrolytic solution as an electrolyte and utilizing oxidation and reduction of lithium has been utilized.
In such a non-aqueous electrolyte battery, a carbon material capable of occluding and discharging lithium ions has been conventionally and widely utilized as its cathode material.
In the case of the non-aqueous electrolyte battery using the carbon material for the cathode, lithium contained in the carbon material used for the cathode reacts with a solvent or the like in the non-aqueous electrolytic solution at the time of preservation, so that self-discharge is induced, and the non-aqueous electrolytic solution is decomposed. As a result, the battery capacity is gradually decreased, and the cycle characteristics are degraded.
As the carbon material used for the cathode as described above, carbon which is not graphitized and graphitized carbon have been conventionally generally used.
When a carbon material having low crystallinity such as carbon which is not graphitized is used, a discharge capacity per unit weight is small, whereby a battery having a sufficient discharge capacity is not obtained. Further, potential at the cathode is gradually increased as discharge is induced, whereby the voltage of the battery is reduced, so as to not obtain a stable voltage. On the other hand, when a carbon material having high crystallinity such as graphitized carbon is used, the discharge capacity per unit weight is increased. However, the potential at the cathode is gradually increased at the end of discharge, whereby the non-aqueous electrolytic solution is decomposed upon reaction on the surface of the carbon material used for the cathode, resulting in degraded cycle characteristics.
In the prior art, a non-aqueous electrolyte battery whose cycle characteristics are improved by adding a non-fluorine polymer mainly composed of butadiene and a fluorine polymer as binders to graphite powder, to hold binding power in a depolarizing mix for the cell as well as to increase binding properties to a copper foil which is a collector, has been proposed, as disclosed in Japanese Patent Laid-Open No. 215761/1994.
Even when the non-fluorine polymer mainly composed of butadiene and the fluorine polymer are thus used as binders, however, a non-aqueous electrolytic solution is still decomposed upon reaction on the surface of graphite, so that the cycle characteristics cannot be sufficiently improved.
Furthermore, in the prior art, an aqueous electrolyte battery whose cycle characteristics and preservation characteristics are improved by mixing a whisker such as a silicon carbide whisker, a silicon nitride whisker, or a potassium titanate whisker with powder of an active material such as a carbon material, as disclosed in Japanese Patent Laid-Open No. 302315/1994, and an aqueous electrolyte battery whose collection efficiency is improved using a carbon material obtained by mixing graphite and coke for a cathode, as disclosed in Japanese Patent Laid-Open No. 84515/1994, for example, have been also developed.
Also in each of the aqueous electrolyte batteries disclosed in the gazettes, it is impossible to sufficiently prevent self-discharge from being induced upon reaction of lithium in the carbon material used for the cathode with a solvent or the like in the non-aqueous electrolytic solution as described above and sufficiently prevent the non-aqueous electrolytic solution from being decomposed, whereby the battery capacity is still reduced at the time of preservation, and the cycle characteristics are still degraded.